Question

Magnesium oxide is a compound of magnesium and oxygen. How do you know that it is a completely different substance from each of the two elements it is made from?

Answer 1

When elements combine in a particular molar ratio, they form a compound. For these types of conversion some external changes should be made like heating, applying pressure, cooling etc. The compounds formed after that have some different physical and chemical properties like change of state (soli, liquid, gas).

Complete answer:
We have many elements in our periodic table which are arranged in increasing atomic numbers. This includes some periodic trends, it means increasing atomic radii on moving down the group or decreasing atomic radii on moving left to right. Similarly there is an increase of electronegativity when we move from left to right that is from hydrogen to neon side. So many compounds formed on the basis of electronegativity difference and also by the tendency to donate electrons or accept electrons.
It is clear when two or more elements combine they form a compound, as the name suggests that it will form by combination. This combination takes place when the elements are in some molar ratio. Elements have their particular position in a periodic table where they stand with appropriate values of electronegativity, electron gain enthalpy like terms.
For magnesium oxide $MgO$ , it is a compound i.e. it is formed by combining magnesium $Mg$ and oxygen ${O_2}$ . Physically this process takes place when magnesium ribbon is burned in excess of oxygen, a greyish ash is produced. Now the question arises that how will we differentiate between them so firstly magnesium element is a solid of silvery white color while greyish ash produced are of magnesium oxide. Chemical properties of both are almost different, magnesium ribbon when dropped in water burns vigorously while magnesium oxide when mixed with water gives magnesium hydroxide and this reaction gets reversed when we apply heat.

Note: Physical properties like color, shape, odour (smell) are different for element and its compound. Similarly chemical properties are also different like their boiling point, melting point etc. We can take an example of water which is formed by the combination of two gases hydrogen and oxygen.